Terahertz waves occupy the middle ground between infrared light waves and microwaves and consist of electromagnetic waves at frequencies approximately between 0.1 and 10 terahertz (THz). The terahertz waves have frequencies corresponding to an intermediate range between radio waves and light waves. The terahertz waves are also called submillimeter waves or far infrared waves.
Frequencies of the terahertz waves correspond to molecular vibration frequencies of large molecules. Especially, the terahertz waves may be very useful for analysis of biological materials with very long molecules. Also, it is advantageous that the terahertz waves are used for analyzing fine properties which are unable to be directly measured using other techniques. Development and researches for various technologies using the terahertz waves are actively in progress in various fields such as security inspection, diagnosis, material analysis, next generation communication and the like.
The main factor that the development of the terahertz technology is more delayed than other electromagnetic wave bands, such as visible rays, infrared rays or microwaves, is the difficulty in generating and detecting terahertz radiation. In recent time, techniques of generating ultra-short terahertz waves using ultra-short laser or electron beam pulses having a time width of picoseconds (10-12 sec) or less are actively being developed. However, the terahertz frequency range still exhibits extremely low generation efficiency as compared with other neighboring electromagnetic wave ranges. This may cause lasers or accelerators to become very large in size in order to obtain high-power terahertz waves. Such high-power terahertz waves are also generated using large accelerators. In order to generate the high-power ultra-short terahertz waves using small-scale accelerators, instead of large accelerators which are high-priced, it is necessary to develop a new technique of generating terahertz waves with improved efficiency. This is one of the most important tasks in the terahertz science and technology field worldwide.
Transition radiation is radiation emitted when a relativistic electron passes through a boundary between two media having different refractive indexes from each other. In general, a thin conducting foil is widely used as the boundary. Incoherent transition radiation shows quite low efficiency of converting kinetic energy of an electron into radiation. When the transition radiation meets a coherent condition, output and generation efficiency may also increase. When a length of an electron beam used for radiation generation becomes shorter than a characteristic wavelength of radiation generated, the output and the generation efficiency may be enhanced by virtue of a coherent effect. Upon use of ultra-short electron beam pulses below picoseconds, the coherent transition radiation may be generated at the terahertz frequency range, and in this case, approximately 10−5 to 10−4 of the electron beam kinetic energy may be converted into the radiation.
High-energy ultra-short terahertz waves reaching up to 100 MW have recently been generated by a large accelerator according to the aforementioned method. Even in this case, the generation efficiency of the terahertz waves is still rather low.